Successfully complete all Hunter secondary missions to unlock the Spike-Quilled Axe. Note: The Hunter is located near the large stone cross in the Grasslands, and he will move halfway through his missions to the Ice Barracks. Once you complete all Hunter secondary missions, return to the first place you met him, and he will give you the Steel-Quill Axe.

Successfully complete the game. Then, go to one of the following locations to find the corresponding character(s) and have a final interaction with them:

Fire Baron: In the jungle directly north of the tree island at the bottom of the last continent. He is to the left of the main road as soon as it turns, traveling south to east.

Infantry/Army: At the Beach Party

Lita: In front of Lars's tombstone at the cemetery. There will be a brief cutscene where you share a moment of silence with her. The moment will last and the time of day will shift until you press [Interact] again.

Mangus: On top of the large sword in Blade Henge. Note: Use the nearby Landmark Viewer.

Ophelia: In front of the large stone cross near The Hunter in the Grasslands (located to the left of the huge sword on the map). Eddie will ask if she wants to make out. A cutscene will play of the two kissing until you press [Interact] again.

Rima: On top of her tower in the jungle

Thunderhogs: At the Kill Master's Lair. You can drink with his bassists and comment on Lemmy's bass playing ability.

Riding the animals

All animals as big as Eddie or larger can be ridden. Hit a creature with a bolt of lightning to stun them. Then, get near them, and press Triangle. While riding them, press X to perform an attack specific to that creature. Raptor Elk and Tullusks are a couple examples of creatures with melee attacks, while the Laser Panther shoots lasers from its eyes. The large wooden horse creatures in the final map area (with large swinging axes for tails) can also be ridden.

Kyle Gass reference

The man at the Mortar missions has the same facial features, hair, and voice as Kyle Gass. Kyle Gass is the lead guitarist and vocalist of Tenacious D and the Godfather of Jack Black's son. -From: Tyler Murphy

Psychonauts reference

In the first cutscene, where you are given the options to toggle the blood gore and swearing, just before Eddie says the line that leads to the option of turning on or off swearing, he sees one of the band members dancing on a very shaky ledge. While that band member is dancing on that ledge, he will be chanting a song that was originally done in Psychonauts by some kids in a few parts, but most notably by Raz himself in the level where you have to fight four wrestlers to get a card. At the end of each match after they were defeated, Raz did the same dance and chant song.

The headbanger that wears the goggles at all the Death Rack missions sounds identical to Raz, but slightly deeper.

Rob Halford reference

Rob Halford plays General Lyonwhite even though his last name is Lita and Lars'.

Trophies

Successfully complete one of the following tasks to get a trophy:

Got a Car and a Date (Bronze): Completed 'Welcome to the Age of Metal'.

Start a Revolution (Bronze): Completed 'Exploited in the Bowels of Hell'.

Thick as a Baby's Arm (Bronze): Completed 'Lair of the Metal Queen'.

Hair Remover (Bronze): Completed 'Battle for Bladehenge'.

French Kiss Instructor (Bronze): Completed 'Pilgrimage of Screams'.

Chicks n' Booze n' Stuff (Bronze): Completed 'Sanctuary of Sin'.

Ran to the Hills (Bronze): Completed 'It's Raining Death'.

Fistfull of Fog (Bronze): Completed 'Dry Ice, Wet Graves'.

No More Tears (Bronze): Completed 'Sea of Black Tears'.

Overkill (Bronze): Completed all hunting secondary missions.

Squeal Like a Chicken (Bronze): Completed all racing secondary missions.